A conventional popular mechanical pencil comprises a shaft cylinder having a tip member, a lead pipe inserted into the shaft cylinder and biased in a retract direction, and a chuck body coupled to a tip of the lead pipe for releasably gripping a writing lead. The lead pipe is advanced by a knocking operation so that the writing lead protrudes from the tip member for writing.
Such a conventional mechanical pencil is used for writing in a state where the writing lead protruding from the tip member is gripped by the chuck body so as not to rotate. Thus, when the mechanical pencil is used for writing with the shaft cylinder grasped by a user in a fixed orientation with respect to a sheet of paper, a lead tip of the writing lead is worn down unevenly due to abrasion, resulting in change of thickness and density of written lines. In order to overcome this, the shaft cylinder should be intermittently rotated during writing. Such an operation is troublesome, and inconvenient in terms of usability.
A mechanical pencil which comprises a chuck unit and a lead rotating mechanism for rotating a writing lead together with the chuck unit by retracting the chuck unit via a pressing force of the writing lead on, for instance, a sheet of paper, is also provided (for example, Patent Documents 1 and 2).
In a mechanical pencil disclosed in Patent Document 1, a chuck body rotating means (lead rotating mechanism) is disposed in a shaft cylinder. By retracting a writing lead while a chuck body opened and closed by a chuck ring grips the writing lead, the chuck body rotating means rotates a so-called chuck unit comprising the chuck body and chuck ring. The chuck body rotating means is comprised of a sliding element, a rotor abutting the sliding element, and a cam on an inner wall surface of the shaft cylinder. The sliding element abuts the back end of the chuck ring opening and closing the chuck body; the rotor moving in association with movement of the sliding element abuts the sliding element; the sliding element, chuck body, and rotor are biased forward; and a writing lead tank is secured to a back part of the chuck body (Patent Document 1: paragraph [0005]). When the writing lead is slightly strongly pressed against a sheet of paper, the sliding element, chuck body, and rotor are retracted against the forward bias. When the press of the writing lead against the sheet of paper is released, the sliding element, chuck body, and rotor rotate while advancing by the forward bias. This rotation causes the chuck body to rotate together with the lead tank, which in turn rotates the writing lead gripped by the chuck body.
In a mechanical pencil disclosed in Patent Document 2, a chuck unit is provided within a shaft cylinder so as to be rotatable and movable forward/backward. The mechanical pencil comprises a rotation-driving mechanism (lead rotating mechanism) for rotation-driving a rotor in association with retraction of the chuck unit by writing pressure of a writing lead. The rotation of the rotor is transmitted to the writing lead via the chuck unit. Since the chuck unit is coupled to a tip of a lead case (Patent Document 2: paragraph [0030]), the rotation-driving mechanism is configured to integrally rotate the chuck unit gripping the writing lead and the lead case.